This invention relates to an electrical connector, and more particularly to an electrical connector having electric contacts on both sides.
In order to inspect performance of semiconductor elements of a ball grid array package having a plurality of electric contacts in the form of a hemispherical protrusion made of solder or the like, for example, a socket connector has been used, which is detachably fitted with the ball grid array package. Such a socket connector includes a desired number of electric contacts to be connected to the electric contacts made of solder of the ball grid array package. Some socket connectors have electric contacts of as many as several hundreds in the largest number case. Their electric contacts are arranges with extremely narrow pitches, typically 0.8 mm. Such electric contacts are often arranged on a flexible printed circuit board.
As described above, several hundreds, of the electric contacts are arranged with extremely narrow pitches. In the case that, for example, there are four hundred electric contacts arranged in twenty rows each having twenty contacts, it is required that a signal from one electric contact located, for example, at the center of them is transmitted through a lead wire leading through one edge of a flexible printed circuit board to an exterior equipment. In such a case, the lead wire must pass between at least nine rows of contacts and its length passing therebetween may become at least 0.8 mm×9=7.2 mm, and at the outermost rows as many as nine lead wires must be arranged between two adjacent electric contacts.
On the other hand, in the case that electric contacts are 0.3 mm in diameter and arranged with a pitch of 0.8 mm, the space in which lead wires are arranged becomes 0.8 mm−0.3 mm=0.5 mm. In order to arrange nine lead wires in such a narrow space, a diameter of the lead wires is 0.5 mm÷19=0.026 mm, assuming that a ratio of lead wire diameter to space is 50%. Therefore, forming a circuit with a good transmission line becomes very difficult because to the space is far too small for the lead wires.
In view of such a difficulty to provide lead wires, the inventors of the present application attempted to use, for surface treatment of electric contacts, the electroless plating which needs no electrical connection to plating electrodes, for the purpose of providing circuits on the opposite surface of a board utilizing through-holes formed therein.
However, the electroless plating is time-consuming. For example, it takes 1 to 1.5 hours for providing plated layers of as little as 5 micrometers on electric contact elements. Therefore, a flexible circuit board must be kept immersed in a plating bath which is alkaline for a long period of time, creating a risk of copper foils on the board being partially peeled or removed. The inventors, therefore, have attempted to obtain plated electric contact elements by electrolytic plating instead of electroless plating.
On the other hand, an electrical connector has been used, which has an elastomer layer or board between the circuit boards and the electronic components. The elastomer or board is made of a silicone rubber or the like having a plurality of fine lead wires embedded therein and extending in the width direction of the elastomer layer. The elastomer layer is further provided on both surfaces with electric contacts connected to their respective lead wires to electrically connect the circuit boards and the electronic components.
With a connector of this kind, in order to ensure the electrical connection between the circuit boards and the electronic components, the electronic components are forced against the circuit boards. For this purpose, the fine lead wires embedded in the elastomer layer extend rectilinearly at right angles to both the surfaces of the elastomer layer, and both the ends of the fine lead wires extend slightly beyond both the surfaces of the elastomer layer to form an electrical connector having electric contacts on both the surfaces as disclosed, for example, in Japanese Patent Application No. 2001-270,521.
In order to reduce the self-inductance of lead wires electrically connecting therebetween for the high speed signal transmission, these lead wires need to be as short as possible in length and as large as possible in diameter and to be formed of a material of high conductivity.
In such an electrical connector, moreover, it is difficult to embed the fine lead wires in an elastomer layer or board in a manner such that the overall lengths of the fine lead wires are exactly embedded in the elastomer layer with its thickness completely coincident with the lengths of the fine lead wires. Consequently, on one side, the ends one ends of the fine lead wires are buried in the elastomer layer, but the other ends extend slightly beyond the surface of the layer. Extruding forces, acting upon the elastomer layer from the fine lead wires are thus different between one surface and the other of the elastomer layer or board, causing the elastomer layer to warp or bend.
In the manufacturing process for such an electrical connector, both surfaces of the elastomer layer are jointed to flexible printed circuit boards each including electric contacts adapted to contact mating contacts, an insulating board formed with through-holes extending therethrough and conductors electrically connecting the electric contacts through the through-holes in a manner such that both the ends of fine lead wires extending slightly beyond both the surfaces of the elastomer layer in which the fine lead wires are embedded are aligned with and extend into the through-holes of the flexible printed circuit boards.
However, if the elastomer layer is warped or bent as described above, there would have been clearances between the ends of the fine lead wires and the insulating board at the through-holes, into which edges of the holes of the elastomer layer embedding the fine lead wires therein would enter the clearances, thereby reducing the contacting surfaces between the fine lead wires and the through-holes of the insulator boards. As a result, the fine lead wires and the flexible printed circuit boards are poorly soldered with a lower solderability, so that during repeated connection action the mating contacts, there is a tendency for the peeled solder metal to cause defective or failed connection of the connector.
While the fine lead wires are being urged after they are brought into the predetermined positions relative to the elastomer layer, an electrical connection between the flexible printed circuit boards and the fine lead wires is often impossible because the shoulders of the fine lead wires and their proximities are covered by the elastomer layer.
Moreover, the heat during the reflow soldering causes the thermal expansion of the flexible printed circuit boards so that circuit boards will warp to move away from the shoulders of the fine lead wires, as a result of which the desired connections become impossible. Even if, the connections overcome the warp, they will be attended by poor solderability with insufficient soldering strength.
Furthermore, it is envisioned that the reflow soldering is performed, while the flexible printed circuit boards are clamped by means of jigs or the like, in order to prevent the flexible printed circuit boards from moving away from the shoulders of the fine leading wires. In the case that the portions to be connected are numerous and arranged in high density, the positions restrained by the jigs are limited to very narrow areas so that frame members of the jigs are inevitably thin rods. Moreover, because the jigs are necessarily exposed to high temperatures under high stresses, the material of the jigs is limited to a metal because of its heat resistance and worked accuracy.
Unfortunately, a jig made of a metal has a great heat capacity, so that in order to raise the temperature of the portions to be soldered to a temperature required for the reflow soldering by the use of heated air, infra-red radiation or the like, heat needs to be supplied at a much higher temperature or for a much longer period of time. However, such a large amount of heat would adversely affect various parts directly or indirectly related to the electrical connector, resulting in damage to these parts.